LONDON - Prime Minister Theresa May bowed to pressure from Brexit supporters in her Conservative Party on Monday, accepting their changes to a customs bill that underpins Britain's departure from the European Union.

May, vulnerable in parliament after losing her party's majority at an ill-judged election last year, has come under fire from both wings of her party over a hard-won Brexit plan, with one ex-minister calling it the "worst of all worlds".

Eurosceptic lawmakers had targeted her government's customs legislation to try to toughen up her plans to leave the EU, but instead of facing them down and fuelling tensions, her spokesman said the government would accept their four amendments.

It was not clear the move would fundamentally change her plans - the changes do little more than to put government policy into law, her spokesman said - but it was a victory of sorts for those lawmakers who say May has betrayed them on Brexit, the biggest shift in British trade and foreign policy for decades.

However, by hardening the language to emphasise that the future collection of duties and taxes by Britain and the EU is on a reciprocal basis, Brexit supporters may have made May's plan less sellable to the bloc.

May denied a suggestion in parliament that her Brexit plan was dead, and her spokesman said the decision to accept those amendments were "consistent" with the white paper policy document ministers agreed earlier this month.

"We've accepted the amendments because we believe they are consistent with the approach that we set out, and in a number of cases it reinforces some of the messages that came in the white paper," the spokesman told reporters.

Where the government might struggle is explaining its acceptance of the demand that the EU must collect tariffs on Britain's behalf, if London is to do the same.

The spokesman said that was met by the government's pursuit of "a mechanism for the remittance of relevant tariff revenues". But one expert, Anand Menon, professor of European politics and foreign affairs at King's College London, said the relationship could never be reciprocal.

"There is no way a government white paper can stipulate that 27 other countries are going to collect our tariffs for us. It makes no sense," he said.

The battle over the amendments to the Taxation (Cross-border Trade) Bill, or customs bill, is unlikely to be the last that May and her team will have to face.

"WORST OF BOTH WORLDS"

May had to fight hard to get the agreement of cabinet ministers at her Chequers country residence earlier this month for her vision for Britain's future ties with the EU. It was then undermined by the resignations of her Brexit minister David Davis and foreign secretary Boris Johnson.

The plan, only a starting point for the second phase of talks with the EU, has come under fire from other eurosceptic lawmakers, who say the proposal to keep close customs ties to the EU betrays her commitment for a clean break with the bloc.

On Monday, the other wing of May's Conservative Party - those lawmakers who want to keep the closest possible ties with the EU after Brexit - spoke up in the voice of former education minister Justine Greening who called for a second referendum.

Greening said such a vote was the only way to break the stalemate in parliament over the best future relationship with the bloc and branded May's plan as "a fudge I can't support. It's the worst of both worlds".

May's spokesman said there would be no second referendum under any circumstances, and restated her position that the Chequers plan was the only way to deliver a Brexit that worked in the best interests of the country.

Another pro-EU lawmaker Dominic Grieve, who has led previous efforts to get the government to soften its Brexit stance, said the party needed to accept compromises "or accept that Brexit cannot be implemented and think again about what we are doing".

For now the impetus lies with the Brexit supporters.

Jacob Rees-Mogg, an arch eurosceptic who proposed the amendments, said he did not expect the bill, or another bill on trade due to be debated on Tuesday, to be blocked outright by the 650-member parliament. Rees-Mogg said that he wanted rather to test the support in parliament for changing her strategy.

"I'm sure Theresa May does not want to split the Conservative Party and therefore she will find that the inevitable consequence of the parliamentary arithmetic is that she will need to change it (the Brexit policy) to keep the party united," Rees-Mogg said.

Reuters